Coating is the process of replacing the gas contacting a substrate, usually a solid surface such as a web, with a layer of fluid, such as a liquid. Sometimes, multiple layers of a coating are applied on top of each other. Often the substrate is in the form of a long continuous sheet, such as a web, wound into a roll. Examples are plastic film, woven or non-woven fabric, or paper. Coating a web involves unwinding the roll, applying the liquid layer to the roll, solidifying the liquid layer, and rewinding the coated web into a roll.
After deposition of a coating, it can remain a liquid such as in the application of lubricating oil to metal in metal coil processing or the application of chemical reactants to activate or chemically transform a substrate surface. Alternatively, the coating can be dried if it contains a volatile liquid, or can be cured or in some other way treated to leave behind a solid layer. Examples include paints, varnishes, adhesives, photochemicals, and magnetic recording media.
Methods of applying coatings to webs are discussed in Cohen, E. D. and Gutoff, E. B., Modern Coating and Drying Technology, VCH Publishers, New York 1992 and Satas, D., Web Processing and Converting Technology and Equipment, Van Vortstrand Reinhold Publishing Co., New York 1984, and include knife coaters.
Knife coating involves passing the liquid between a stationary solid member, a knife, and the web so that the clearance between the knife and the web is less than twice the thickness of the applied liquid layer. The liquid is sheared between the web and the knife, and the thickness of the layer depends to a great extent on the height of the clearance. For many materials and operating constraints, knife coaters have the advantage over other applicators of providing smooth coatings, free of waves, ribs, or heavy edges. The web can be supported behind by a roller. The advantage provided by a backup roller is to eliminate the dependence of the coating process upon variations in longitudinal tension across the web, which are common with paper and plastic film substrates. Alternatively, the knife coater can apply a coating directly to a roller, which subsequently transfers the coating to the web.
One feature which distinguishes various knife coaters is the means by which liquid is introduced to the knifing passage. Gravity fed knife coaters, shown in FIG. 1, receive liquid from an open pool contained against the web by a hopper. Large volumes are required to distribute the liquid evenly across wide web widths, requiring substantial cleanup and large material losses during changeover. Also, particles and bubbles can lodge in the gap between the knife and the substrate and produce streaks in the coating, and air entrainment between the liquid layer and the web is difficult to control.
Film fed knife coaters, shown in FIG. 2, receive liquid from a layer applied to the web by some other means, but not yet with the desired thickness, uniformity, or smoothness. Any excess material runs off the knife and is collected for recycle. However, handling the recycle stream without entraining air or debris is difficult. Also, evaporation of the liquid due to the expansive fluid-air interfaces and long residence time can change material properties and expose human operators to harmful vapors. Additionally, if the initial coating layer is applied with gross imperfections, traces of the imperfections are likely to remain after the knifing passage.
Die fed knife coaters, shown in FIG. 3, receive liquid from a narrow slot, which in conjunction with an upstream manifold, distributes evenly across the web the flow feeding the knifing passage. The die includes two plates sandwiched together with a shim or a depression in one plate forming the slot passage. Cleaning the coater, or changing coating widths requires disassembly of the two plates. Moreover, particles and bubbles can lodge in the gap between the knife lip and the web, because there is no other exit for them, producing streaks in the coating. Also, machine direction uniformity of the coating is sensitive to line and pump speed changes because the liquid has no other exit except onto the web (except with extreme overfeeding in which case excess material is squeezed out the upstream passage between the die lips and the web).
Trough fed knife coaters, shown in FIGS. 4A and 4B, receive liquid from a wide slot, or trough, which is fed by a narrow slot and manifold to provide even flow distribution across the web. Cleaning these coaters requires disassembly of the two plates which form the slot and manifold. The coater in FIG. 4A accumulates particles and gels in the trough, which eventually become lodged in the knifing passage to produce streaks. The coater in FIG. 4B overflows on the upweb side of the coater. The overflow is recycled, but is susceptible to entrainment of debris and air.